1. Field
Various embodiments of the invention pertain to wireless communication systems. At least one embodiment of the invention pertains to a system and method of assisting in the acquisition of an access terminal by a target base station during a handoff.
2. Background
Wireless communication networks enable communication devices, such as access terminals, to transmit and/or receive information while on the move. These wireless communication networks may be communicatively coupled to other public or private networks to enable the transfer of information to and from the access terminals. Such communication networks typically include a plurality of base stations which provide wireless communication links to access terminals. The base stations may be stationary (e.g., fixed to the ground) or mobile (e.g., mounted on satellites, etc.) and positioned to provide wide area coverage as the access terminal travels across different coverage areas.
As an access terminal moves around, its communication link with a current base station may degrade. In this situation, the access terminal attempts to switch or connect with another base station for a better quality communication link while its current link is still active. This process of establishing a communication link with another base station is referred to as a “handoff.” Soft handoffs and hard handoffs are two commonly used types of handoffs. A soft handoff is one where a new communication link with a new base station is established before the existing communication link is terminated. In a hard handoff, an existing communication link is typically terminated before a new communication link is established.
The handoff process typically encounters the problem of maintaining a reliable communication link with the wireless communication network while switching base stations. At some point during a soft handoff, a target base station attempts to acquire a “pilot” signal coming from an access terminal that is trying to switch the communication link from its current base station to a new target base station. One approach to acquiring an access terminal signal is to search for a known portion of its waveform. Thus, to assist with this demodulation and acquisition, a known signal, commonly referred to as a “pilot”, is transmitted by the access terminal together with its information bearing signals.
The uplink (i.e., communication link from an access terminal to a base station) of many cellular communication systems employ Code Division Multiple Access (CDMA) as a means for simultaneously sharing the same frequency bandwidth among all the access terminal transmitting to a particular base station. Because of the non-orthogonal nature of CDMA, this multiplexing scheme relies on power control techniques devised to regulate the amount of interference that each individual transmission is causing to the rest of the access terminals being demodulated by a base station. Generally speaking, power control regulation has each access terminal transmit the minimum amount of power necessary to meet its grade of service (GOS) requirements. An access terminal communicating with a current base station minimizes its transmission power for a given communication performance level, which results in system resources being allocated more efficiently. In this manner, the interference caused to other access terminals is minimized and the overall system capacity is maximized.
Taking as a baseline a base station equipped with a single receiving antenna and simple digital processing that only attempts to implement a channel matched filter for each access terminal, a number of improvements are possible that increase uplink capacity with respect to such baseline. For instance, various forms of multiple antenna processing, uplink channel equalization, and interference cancellation may be implemented. As noted above, an increase in capacity can be achieved by a reduction of the access terminal's transmitted power needed to achieve a given grade of service when such techniques are in place. These enhancements are, in essence, more efficient ways of utilizing the power of the received signals in the demodulation and decoding process, resulting in a reduction of the needed power.
In their most simple and widely proposed forms, the capacity-improving techniques mentioned above are applied after the access terminal's pilot signal has been acquired. This causes a problem during the handoff process however. During a handoff, a target base station acquires an access terminal's pilot signal without the named performance enhancements. This means that the access terminal is transmitting a weaker pilot signal to the target base station during handoff, making it harder for the target base station to acquire the signal. This may impact the length of time it takes to acquire a signal.